1. Field of the Invention
The invention relates to new polymeric azomethines and to process for preparing the same by condensation of a substituted benzidine and a dialdehyde. They are of an intense yellow or yellow-green to orange color. They have an extraordinarily high thermal stability and are suitable for use as pigments in various plastics.
2. Discussion of the Prior Art
It has long been known to prepare azomethines, which are also called "Schiff bases" in the literature, and have the general formula ##STR4## from primary amines and aldehydes or ketones with the liberation of water (e.g., benzalaniline according to Schiff, Liebigs Annalen der Chemie, Supplement 3, 353): ##STR5## This reaction can also be applied to bifunctional and polyfunctional aldehydes, resulting in the formation of polymeric or crosslinked polymeric azomethines with the liberation of more water molecules. Aromatic aldehydes and amines usually result in colored compounds.
Aromatic polymeric azomethines are described by Manecke et al in Makromol. Chem., (1970), 133, pp. 61-82. The structural units of these polymers are derived from 2,5-dihydroxyterephthalic dialdehyde and, for example, unsubstituted benzidine. They are prepared by the condensation of the dialdehyde with the amine in glacial acetic acid as solvent, with the addition of sodium acetate. The reaction mixture is refluxed, and the reaction product that precipitates is suction filtered. For purification, the product must be extracted for eight hours with water and a mixture of methanol and acetone.
Disadvantageous in this procedure is the use of acetic acid as solvent, and the fact that the products obtained are impure and have to be subjected to an additional, tedious purifying operation. In spite of these drastic circumstances, the yields are only between 40% and 80% of the theory.
Furthermore, a method is described in German Offenlegungsschrift No. 2,601,821 for the preparation of polyazomethines by the condensation of terephthalic dialdehyde and phenylene diamines, among others. The starting products are reacted in the gaseous phase, and are used immediately following their preparation for the sealing of objects. This method of procedure has the disadvantage that it has to be performed at relatively high temperatures, with the attendant expenditure of energy, in correspondingly expensive apparatus. Since it is very strongly application-related, it is not suitable for the production of larger and more widely useful amounts.
There is a strong demand on the plastics market for yellow pigments having good thermal stability, color-fastness and no tendency to bleed. The problem therefore existed of preparing pigments having these properties from substituted benzidines and aromatic aldehydes. Such pigments must be thermally stable so that they can withstand all fabrication processes without harm, and they must have no tendency to bleed. The problem also existed of finding the correct selection of the substituents on the benzidine and aldehyde benzene nuclei and to discover a gentle method for the preparation of these new compounds. Such a method should make it possible, furthermore, to achieve a high transformation, and to separate any starting products still present, or any newly formed by-products, by simple means. It should also be one which permits the solvents re-use, so as to minimize the amount of labor required and the attendant risk of degradation.